NAME
rrdgraph_data - preparing data for graphing in rrdtool graph
SYNOPSIS
DEF:<vname>=<rrdfile>:<ds-name>:<CF>[:step=<step>][:start=<time>][:end=<time>][:reduce=<CF>]
VDEF:vname=RPN expression
CDEF:vname=RPN expression
DESCRIPTION
These three instructions extract data values out of the RRD files,
optionally altering them (think, for example, of a bytes to bits
conversion). If so desired, you can also define variables containing
useful information such as maximum, minimum etcetera. Two of the
instructions use a language called RPN which is described in its own
manual page.
Variable names (vname) must be made up strings of the following
characters "A-Z, a-z, 0-9, -,_" and a maximum length of 255 characters.
When picking variable names, make sure you do not choose a name that is
already taken by an RPN operator. A safe bet it to use lowercase or
mixed case names for variables since operators will always be in
uppercase.
DEF
DEF:<vname>=<rrdfile>:<ds-name>:<CF>[:step=<step>][:start=<time>][:end=<time>][:reduce=<CF>]
This command fetches data from an RRD file. The virtual name vname can
then be used throughout the rest of the script. By default, an RRA
which contains the correct consolidated data at an appropriate
resolution will be chosen. The resolution can be overridden with the
--step option. The resolution can again be overridden by specifying
the step size. The time span of this data is the same as for the graph
by default, you can override this by specifying start and end.
Remember to escape colons in the time specification!
If the resolution of the data is higher than the resolution of the
graph, the data will be further consolidated. This may result in a
graph that spans slightly more time than requested. Ideally each point
in the graph should correspond with one CDP from an RRA. For instance,
if your RRD has an RRA with a resolution of 1800 seconds per CDP, you
should create an image with width 400 and time span 400*1800 seconds
(use appropriate start and end times, such as "--start
end-8days8hours").
If consolidation needs to be done, the CF of the RRA specified in the
DEF itself will be used to reduce the data density. This behavior can
be changed using ":reduce=<CF>". This optional parameter specifies the
CF to use during the data reduction phase.
Example:
DEF:ds0=router.rrd:ds0:AVERAGE
DEF:ds0weekly=router.rrd:ds0:AVERAGE:step=7200
DEF:ds0weekly=router.rrd:ds0:AVERAGE:start=end-1h
DEF:ds0weekly=router.rrd:ds0:AVERAGE:start=11\:00:end=start+1h
VDEF
VDEF:vname=RPN expression
This command returns a value and/or a time according to the RPN
statements used. The resulting vname will, depending on the functions
used, have a value and a time component. When you use this vname in
another RPN expression, you are effectively inserting its value just as
if you had put a number at that place. The variable can also be used
in the various graph and print elements.
Example: "VDEF:avg=mydata,AVERAGE"
Note that currently only aggregation functions work in VDEF rpn
expressions. Patches to change this are welcome.
CDEF
CDEF:vname=RPN expression
This command creates a new set of data points (in memory only, not in
the RRD file) out of one or more other data series. The RPN
instructions are used to evaluate a mathematical function on each data
point. The resulting vname can then be used further on in the script,
just as if it were generated by a DEF instruction.
Example: "CDEF:mydatabits=mydata,8,*"
About CDEF versus VDEF
At some point in processing, RRDtool has gathered an array of rates
ready to display.
CDEF works on such an array. For example, CDEF:new=ds0,8,* would
multiply each of the array members by eight (probably transforming
bytes into bits). The result is an array containing the new values.
VDEF also works on such an array but in a different way. For example,
VDEF:max=ds0,MAXIMUM would scan each of the array members and store the
maximum value.
When do you use VDEF versus CDEF?
Use CDEF to transform your data prior to graphing. In the above
example, we'd use a CDEF to transform bytes to bits before graphing the
bits.
You use a VDEF if you want max(1,5,3,2,4) to return five which would be
displayed in the graph's legend (to answer, what was the maximum value
during the graph period).
If you want to apply 'complex' operations to the result of a VDEF you
have to use a CDEF again since VDEFs only look like RPN expressions,
they aren't really.
SEE ALSO
rrdgraph gives an overview of how rrdtool graph works. rrdgraph_data
describes DEF,CDEF and VDEF in detail. rrdgraph_rpn describes the RPN
language used in the ?DEF statements. rrdgraph_graph page describes
all of the graph and print functions.
Make sure to read rrdgraph_examples for tips&tricks.
AUTHOR
Program by Tobias Oetiker <tobi@oetiker.ch>
This manual page by Alex van den Bogaerdt <alex@vandenbogaerdt.nl> with
corrections and/or additions by several people